1. Field of the Invention
The present invention relates generally to a pressure sensor, and in particular to a resonant microbeam pressure sensor having a polysilicon microbeam resonator attached to a portion of the sensor diaphragm and a resonator beam and having an optical fiber directing a light beam to the resonator beam to read the resonance mode optically.
2. Description of the Related Art
A pressure sensor is disclosed in U.S. Pat. No. 5,808,210 in which a micromechnical sensor has a polysilicon beam that is an integral part of the diaphragm. The resonant frequency of the beam is a direct result of the pressure applied to the external surface of the diaphragm. Fabrication of this resonant microbeam sensor requires no backside wafer processing, and involves a process and layout independent of wafer thickness for high yield and robustness. Both the diaphragm and the resonant beam are formed from polysilicon. The sensor may have more than one resonant beam. The sensor beam or beams may be driven and sensed by electrical or optical mechanisms. For stress isolation, the sensor may be situated on a cantilevered single crystal silicon paddle. The sensor may be recessed on the isolating die for non-interfering interfacing with optical or electrical devices. The sensor die may be circular for ease in mounting with fiber optic components. The contents of the U.S. Pat. No. 5,808,210 are incorporated herein by reference.
FIG. 2 of the present application sets forth an example of an earlier embodiment of a resonant beam pressure sensor. The FIG. 2 shows a thin film resonant microbeam absolute pressure sensor 10 in which a beam 12 is held by posts 13 inside a shell 14. The shell 14 is provided on a substrate wafer 16 with a vacuum, or at least a partial vacuum, inside the shell 14. The substrate 16 has a photodiode, or p-n junction, 18 formed on a top surface thereof within the shell 14. A Fabry-Pérot resonant cavity is formed within the shell 14, including a first portion between the beam 12 and the inside of the top of the shell 14 and a second portion of the cavity between the resonant beam 12 and the top of the substrate 16. An optical fiber 20 which is positioned above the shell 14 at the front side of the sensor directs light onto the beam 12 where it encounters the resonant cavity and resonates at a frequency. The resonating beam 12 reflects light back into the optical fiber 20 which transmits the modulated light beam to a light sensor. Changes in pressure result in corresponding changes in the resonant frequency of the beam, so that the pressure is sensed. This device is termed a shell coupled pressure sensor.
In the pressure sensor shown in FIG. 2, the light beam from the optical fiber 20 is transmitted through the medium to be sensed, so that the medium under pressure must be transparent for the device to work properly. If the medium is not transparent, some or all of the light leaving the optical fiber 20 is scattered or absorbed before it can bounce off of the beam and re-enter the fiber. This results in degraded performance of the sensor, or even to the point of the sensor being unable to obtain a pressure reading at all.
A further problem is that of packaging the optical fiber 20 with the pressure sensor 10. The packaging must hold the fiber 20 close to the resonant beam 12 to get a good optical coupling; yet the spacing must be sufficient that the medium to be sensed has access to the shell 14 of the pressure sensor 10.
In FIG. 3, the fiber 20 and sensor shell 14 are set at a fixed distance from one another by spacers 22. Openings 24 in the spacers 22 permit the medium to be sensed to access the sensor chamber 26 while maintaining the spacing of the optical fiber 20 from the resonant beam 12. This arrangement addresses the issue of positioning of the fiber and sensor, but presents considerable difficulties in micromachining. Further, the medium to be sensed may become trapped in the sensor space 26 and can foul the optical window at the end of the fiber 20. Any dirt or foreign matter in the medium to be sensed may accumulate on the optical elements of the sensor and fiber and result in deterioration in performance and eventually to complete loss of function.